DIAGNOSIS Your device is connected to the far-away main unit instead of the nearby satellite. This is a client steering or band issue.
FIX First, verify the connection. In the Deco app, tap the device in question. Check which Deco node it's connected to. If it's the wrong one, toggle the device's WiFi off and on while in the slow room. It should grab the closer satellite.
IF THAT DIDN'T WORK The satellite's wireless backhaul is weak. Improve its connection to the main unit. Move it halfway between the main unit and the problem room. Ensure it's not in a cabinet or behind a TV. For a guaranteed fix, connect the satellite to the main unit using an Ethernet cable for a wired backhaul.
HEADS UP Speed tests from a satellite will always be lower than from the main unit if using wireless backhaul. That's normal. Consistent slowdowns for a specific device are usually the device's own WiFi adapter, not the Deco.